Recombinant GM-CSF has been recently shown to prolong survival of elderly patients with acute myeloid leukemia (AML) by reducing the rate of induction therapy-related mortality. In a prospective, randomized, placebo-controlled, double-blind, multicenter study conducted by the Eastern Cooperative Oncology Group in the United States, granulocyte-macrophage colony-stimulating factor (GM-CSF) was given only to those patients who had hypocellular or remission marrow on day 10 of one or two cycles of standard induction therapy. Although the administration of GM-CSF significantly reduced a wide range of adverse events, the main benefit of this cytokine seems to be mediated by a reduction in sepsis. A similarly designed study, conducted by the Southwest Oncology Group in a directly comparable AML patient population with use of granulocyte colony-stimulating factor (G-CSF) as the supportive cytokine, showed no survival benefit and no reduction in the rates of serious or lethal sepsis. In most current clinical situations, GM-CSF and G-CSF are indistinguishable both in terms of efficacy and toxicity. GM-CSF and G-CSF have very different impacts on the survival of patients with AML. The stimulation of monocyte-macrophage function and proliferation by GM-CSF may mediate the selective benefit of GM-CSF in patients with AML and stem cell transplants. GM-CSF merits further study as therapy for and/or protection against opportunistic sepsis in patients with cancer and will be included in a number of International Oncology Study Group protocols.